sample is taken with every inhalation cycle and is averaged with all other samples taken over the most recent ten-second period. After regular operation begins, samples are averaged over the most recent twenty-second period. ΔP (Delta P) The value displayed in the ΔP (pressure difference) window represents the difference between the PIP value and the PEEP value. Servo pressure The servo pressure display indicates the amount of pressure the machine must generate internally in order to achieve the PIP appearing in the servo-display. Its value can range from 0—20 psi (0—137.9 kPa). If the PIP sensed or approximated at the distal tip of the tracheal tube deviates from the desired PIP, the machine automatically generates more or less internal pressure in an attempt to compensate for the change. The servo-pressure display keeps the operator informed. The servo display is a general clinical indicator of changes in the compliance or resistance of the patient's lungs, as well as loss of lung volume due to tension pneumothorax. High-frequency oscillatory ventilation In HFOV the airway is pressurized to a set mean airway pressure (called continuous lung-distending pressure) through an adjustable expiratory valve. Small pressure oscillations delivered at a very high rate are superimposed by the action of a “loudspeaker” oscillator membrane. HFOV is often used in premature neonates with respiratory distress syndrome who fail to oxygenate appropriately with lung-protective settings of conventional ventilation. It has also been used in ARDS in adults, but two studies (the OSCAR and OSCILLATE trials) showed negative results for this indication. Parameters that can be set in HFOV includes the continuous lung-distending pressure, oscillation amplitude and frequency, I:E ratio (positive-oscillation/negative-oscillation ratio), fresh gas flow (called bias flow), and FiO2. Increases in continuous lung-distending pressure and FiO2 will improve oxygenation. Increases in amplitude or fresh gas flow and decreases in frequency will improve ventilation. High-frequency percussive ventilation HFPV — High-frequency percussive ventilation combines HFV plus time cycled, pressure-limited controlled mechanical ventilation (i.e., pressure control ventilation, PCV). High-frequency positive pressure ventilation HFPPV — High-frequency positive pressure ventilation is rarely used anymore, having been replaced by high-frequency jet, oscillatory and percussive types of ventilation. HFPPV is delivered through the endotracheal tube using a conventional ventilator whose frequency is set near its upper limits. HFPV began to be used in selected centres in the 1980s. It is a hybrid of conventional mechanical ventilation and high-frequency oscillatory ventilation. It has been used to